CAPACITALK No. 105

SIZING CONTACTORS FOR CAPACITOR SWITCHING

Most of the major contactor manufacturers spend a lot of time testing and retesting their contactors for all types of applications, capacitors being one of them. So if you call the manufacturer of the contactor you are using, they should be able to guide you in finding the right size contactor for the size of capacitor you are using.

Also, there is a "rule of thumb" you can use to size the contactor for the capacitor you are using. In today's factory environment, we must not only consider the nominal current of the capacitor, but also some harmonic currents. So for this reason, and because the capacitor tolerance allows it, we will use a factor of 10% times nominal current. (1.1In, where In = nominal current). Then we will take into account the usual 135% rating used for wiring and disconnect sizing (1.35In).

Therefore, it would be best (as a rule of thumb) to size the contactor used for switching the capacitor to be rated to carry a thermal current (Ith) of 1.49 (1.1 x 1.35) times the nominal current of the capacitor, as shown in the formula below.